Calibration cuvette

ABSTRACT

A calibration cuvette apparatus for storing and calibrating a chemical sensor in a tonometered solution includes an upper cuvette section and a lower cylindrical valve section for alternately sealing the cuvette section and admitting a gas mixture to the cuvette section. The upper cuvette section has relatively narrow diameter upper and lower ends on either side of a wide diameter middle portion. The cuvette section is adapted to receive the sensor and retain the sensor in a fluid bath in the cuvette section so that the chemical sensor portion is disposed approximately in the center of the wide middle portion of the cuvette section. The chamber formed within the cuvette section is thus shaped so as to maintain the sensor in a position in the fluid bath so that any gas bubbles within the chamber will not dry the sensor. A lower end portion of the cuvette section includes a gas communication inlet for introducing gas into said chamber to equilibrate the tonometered buffer solution.

This is a continuation, division of application Ser. No. 07/565,463,filed Aug. 10, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to calibration of analytical chemistrydevices, and more particularly relates to a device for calibratingsensors for measuring gas concentrations and pH of a fluid.

2. Description of Related Art

In modern medicine, measurement of acidity (pH), and oxygen and carbondioxide levels in the blood has become an important factor in thedetermination of the respiratory status of a patient. Althoughelectrodes have been developed which are capable of measuring theseblood factors in fluids, such electrodes are of limited use inmeasurement of in vivo blood pH levels. Optical sensors called "optodes"have been developed for taking intravascular measurements of acidity andother blood analytes such as oxygen and carbon dioxide. Such opticalsensors typically include a fluorescent indicator dye placed over thetip of an optical fiber and covered by a membrane which is permeable tothe chemical of interest.

It is frequently desireable to keep such sensors wet prior to use in anaqueous, tonometered buffer solution which is isotonically adjusted tomatch the ionic strength of the fluid of interest, such as blood. Suchsensors must also be sterilized, such as in an autoclave, before theyare used intravenously. The autoclaving process can cause pressurebuildup in the buffer solutions, placing unusual stresses on the fluidcontainer in which the sensor is sterilized to cause leakage of thecontainer. It is also desirable to calibrate such sensors before use,and frequently several times daily, using tonometered sample liquidswith known levels of the analyte of interest. One method of preparing anappropriate tonometered buffer solution involves bubbling a prepared gasmixture, such as of CO₂, O₂, and N₂ through the solution untilequilibration of the gas mixture in the solution occurs. However, it hasbeen found that drying of the membrane of the chemical sensor can occurwhere gas bubbles come in contact with the chemical sensor, affectingthe performance of the sensor. It is also useful to provide a bio-filterin the gas bubbling apparatus to filter the gas mixture before it entersthe solution, but it has been found that such filters can become cloggedif exposed to the buffer solution for extended storage periods.

Accordingly, there remains a need for an apparatus that will allowstorage of the sensor in an appropriate fluid to protect the sensor fromdrying out, that will provide a way of isolating the bio-filter frombecoming clogged during a period of storage of the sensor in the fluid,and that will provide for good sealing of the sensor and fluid in theapparatus for the internal pressure which builds up in the fluid duringthe autoclaving process.

SUMMARY OF THE INVENTION

Briefly and in general terms, a calibration cuvette apparatus accordingto the present invention comprises an apparatus for storing andcalibrating a chemical sensor in a tonometered buffer solution. Theapparatus includes an upper cuvette section and a lower cylindricalvalve section for sealing the cuvette section in one valve position,while allowing the admission of a gas mixture to the cuvette section inthe other valve position. The upper cuvette section has relativelynarrow diameter upper and lower ends on either side of a wide diametermiddle portion. The cuvette section is adapted to hold the chemicalsensor in a fluid bath in the cuvette section so that the chemicalsensor is disposed approximately in the center of the wide middleportion of the cuvette section. The chamber formed within the cuvettesection is thus shaped so as to maintain the sensor in a position in thefluid bath so that any gas bubbles within the chamber will not dry thesensor. A lower end portion of the cuvette section includes a gascommunication inlet for introducing gas into the chamber to equilibratethe tonometered buffer solution.

The lower, generally tubular valve section is preferably formedintegrally with the cuvette section with an axis extending perpendicularto the longitudinal axis of the cuvette section. A generally cylindricalelastomeric valve plug is disposed within the valve chamber, and isslidable between a first sealing position and a second gas communicationposition. The valve plug includes a gas communication channel with aninlet port at one of the ends of the valve plug and a gas outlet port atthe outer circumference of the valve plug adapted to be aligned to be incommunication with the cuvette gas communication inlet when the valveplug is in the gas communication position. The cuvette section ispreferably formed in the shape of an elongated dual frustrum, with thecuvette section having an upper frustoconical portion and a lowerfrustoconical portion, with the wide diameter portion of the cuvettesection located at the wide diameter portions of the two frustoconicalportions. The gas communication channel also preferably includes afilter and a sparger for filtering the gas and dispersing the gasbubbles evenly within the cuvette section as it is introduced into thechamber of the cuvette.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the calibration cuvette apparatus of theinvention; and

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As is shown in the drawings which are provided for purposes ofillustration, and not by way of limitation, the invention is embodied ina calibration cuvette apparatus for storing and calibrating a chemicalsensor in a tonometered solution. The apparatus has a unique shape whichallows for storage and calibration of the sensor in an appropriatesolution, protecting the sensor from extended contact with bubbles inthe solution which could otherwise dry a portion of the sensorsufficiently to affect the performance of the sensor. The apparatus alsoprovides a bio-filter for gas to be infused in the tonometered solution.The apparatus also includes a valve section for isolating the bio-filterfrom becoming clogged during a period of storage of the sensor in thesolution and sealing the sensor and solution in the cuvette section ofthe apparatus during autoclaving of the apparatus.

Referring to FIG. 1, the present invention is embodied in a calibrationcuvette apparatus 10 having a novel shaped cuvette section 11 adapted toreceive a catheter having a chemical sensor portion. The general shapeof the cuvette section is that of a dual frustrum designed to preventthe chemical sensor from contacting any bubbles that may form within atonometered buffer solution in the cuvette section, to keep the chemicalsensor of the catheter wet, in any position of the calibration cuvetteapparatus, when the chemical sensor is inserted into the approximatemiddle of the cuvette section. The calibration cuvette apparatus alsoincludes a lower generally cylindrical valve section 12 formedintegrally with the housing 14 of the upper cuvette section. The housingof the calibration cuvette apparatus is preferably comprised of glass,in order to allow the retention of various gas mixtures, particularly inorder to facilitate long term intravenous blood gas catheter storage inthe calibration cuvette apparatus. Alternatively, it may be possible toform the housing of the calibration cuvette from a variety of plasticswhich may be suitable for retaining different gas mixtures.

As further illustrated in FIG. 2, the upper dual frustrum cuvettesection is generally elongated and tubular in its side aspect, having avertical, longitudinal axis 16. An aperture 18 is provided at theextreme upper luer end 20 for receiving the chemical sensor catheter.The upper end also preferably includes an upper cylindrical neck portion24 connecting the mouth 22 of the luer end with the narrow upper end 28of the upper frustoconical portion 26 of the cuvette section. The upperfrustoconical portion gradually enlarges downwardly to form a relativelywider lower end 30 of the upper frustoconical portion contiguous withthe wide diameter middle portion 32. The middle portion 32 is preferablyformed in the shape of a short cylindrical tube, and extends downwardlyto be contiguous with the wide diameter upper section 36 of the lowerfrustoconical portion 34 of the cuvette section. The diameter of thelower frustoconical portion gradually decreases downwardly to the narrowdiameter and lower end 38 of the lower frustoconical portion.

The upper cuvette section thus forms a hollow, inner chamber 40 forreceiving the chemical sensor catheter, such as an intravascular bloodgas sensor, and maintaining the position of the chemical sensor in theapproximate center of the widened middle portion of the upper cuvettesection, in a bath of buffer solution. The narrow diameter lower end ofthe frustoconical portion of the cuvette section includes a loweraperture 42, which serves as a gas communication inlet for introducinggas mixtures formulated as desired into the buffer solution to maintaina desired proportion of dissolved gases in the solution for purposes ofstorage and calibration of the chemical sensor.

The lower cylindrical valve section includes a lower valve chamber 44formed in the housing 46 of the cylindrical valve section, which ispreferably formed integrally with the housing of the upper cuvettesection. The valve section is generally cylindrical, having a horizontallongitudinal axis extending perpendicular to the vertical, longitudinalaxis of the upper cuvette section. A generally cylindrical elastomeric,piston-type plug 48 is preferably disposed coaxially within the lowervalve chamber, and is slidable within the valve chamber between thesealing position which is illustrated in FIG. 2, and a gas communicationposition. Although the plug is preferably formed of an elastomer, suchas rubber or polyurethane, to form a seal at the cuvette sectionopening, the plug may be formed of other materials such as plastic ormetal, with appropriate seals. In order to facilitate alignment of theelastomeric plug with the lower, gas communication aperture in the uppercuvette section in each of these two valve positions, the inner surfaceof the cylindrical valve housing includes one or more, and preferablytwo, alignment channels or grooves 50 adapted to receive correspondingribs or ridges 52 on the elastomeric plug, extending in a longitudinaldirection aligned with the axes of the valve chamber and elastomericplug. Alternatively such grooves could be placed on the plug, and ridgeson the inner wall of the valve chamber.

The elastomeric plug preferably also includes a gas communicationchannel 56 for receiving the specially formulated gas mixture to beintroduced into the upper cuvette section. The gas communication channelincludes a gas communication inlet 54 at one of the longitudinal ends ofthe elastomeric plug for receiving gas from an external supply, and anoutlet end on the circumference of the elastomeric plug, whichpreferably includes a filter chamber 58. A biofilter 60 is preferablydisposed in the filter chamber for filtering out undesirable particulatematter which may be carried along from an exterior gas supply line, andthe filter chamber also preferably includes a frit, such as a thin glassfrit 62 for sparging gas into the lower gas communication aperture 42 ofthe upper cuvette section, when the gas communication channel of theelastomeric plug is placed in its gas communication position alignedwith the gas communication aperture of the cuvette section. Othermaterials which may be adapted for use in frit 62 include ceramics,polymerics or the like. Closely adjacent to the filter chamber on thecircumference of the elastomeric plug is a sealing area 64 on thecircumference of the elastomeric plug, adapted to be aligned with theopening in the cuvette section to seal the cuvette section in the valvesealing position during autoclaving and storage. Support structures,such as the fins 66a and 66b, may also be formed along with the housingto connect the upper housing of the cuvette section with the lowerhousing of the valve section, to provide added support and strength tothe narrow diameter connection of the cuvette section to the cylindricalvalve section.

It will be apparent to those skilled in the art from the foregoing thatthe calibration cuvette apparatus will maintain a seal around thesealing area of the elastomeric plug against the internal pressure ofthe cuvette solution caused by the elevated temperatures which occurduring autoclaving, and that the biofilter will be isolated andprotected during autoclaving and storage when the elastomeric plug ofthe valve is disposed in its sealing position. It is also significantthat the elastomeric plug of the valve section can slide to a gascommunication position to align the biofilter and sparger frit with theopening in the cuvette section to the solution through which a gasmixture can be bubbled when the chemical sensor in the cuvette sectionis to be calibrated. Due to the novel dual frustrum shape of the cuvettesection, gas bubbles in the solution in the cuvette section will eitherdissolve in the solution, rise to the upper end of the cuvette sectionwhen the cuvette section is disposed vertically, or rise to the widediameter middle section of the cuvette section if the cuvette section isplaced horizontally. Thus, a chemical sensor 68 placed in theapproximate middle of the cuvette section will be protected from any gasbubbles in the solution within the cuvette section, and drying of thechemical sensor, either during calibration or storage will be prevented.It is also significant that the biofilter and sparger frit in theelastomeric plug can be kept isolated from the solution in the uppercuvette section during a storage period and before calibration of achemical sensor, so that particulate matter within the solution will notclog the sparger or biofilter.

While particular forms of invention have been illustrated and described,it will be apparent that various modifications can be made withoutdeparting from the spirit and scope of this invention. Accordingly, itis not intended that the invention be limited, except as by the appendedclaims.

What is claimed is:
 1. A method of calibrating a sensor formed on adistal portion of an optical fiber, which comprises the steps of:forminga generally cylindrical chamber having a diameter which is larger in thecentral portion than in respective ends thereof; providing valve meansat a lower end of said chamber to selectively introduce a gas into saidchamber; introducing an optical fiber sensor into said chamber so that adistal end of said sensor is located in the portion of the chamber withthe largest diameter; filling said chamber with a fluid; and introducinga quantity of gas into said chamber, through said valve means.